1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in image forming apparatus such as copiers, facsimiles, laser printers and direct digital platemakers. In addition, the present invention relates to a method for manufacturing the electrophotographic photoreceptor and an image forming apparatus using the electrophotographic photoreceptor.
2. Discussion of the Background
Electrophotographic image forming methods are widely used for copiers, facsimile machines, laser printers, direct digital printing platemakers, etc. Such electrophotographic image forming methods typically include the following processes:    (1) charging a photoreceptor (charging process);    (2) irradiating the photoreceptor with imagewise light to form an electrostatic latent image thereon (imagewise light irradiation process);    (3) developing the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor (developing process);    (4) transferring the toner image onto a receiving material such as paper optionally via an intermediate transfer medium (transfer process);    (5) fixing the toner image on the receiving material, for example, upon application of heat and pressure thereto (fixing process); and    (6) cleaning the surface of the photoreceptor (cleaning process).
The requisites for such photoreceptors are as follows:    1. being able to be charged in a dark place such that the photoreceptors have a proper electric potential;    2. being able to maintain a charge in a dark place (i.e., the charge decay is little in a dark place); and    3. being able to decay the charge upon irradiation of imagewise light thereto.
In addition, recently the photoreceptors are required to have the following properties:    4. having low manufacturing costs;    5. hardly causing pollution; and    6. being able to stably produce good images for a long period of time without causing undesired images.
As the electrophotographic photoreceptor (hereinafter referred to as a photoreceptor) for use in such electrophotographic image forming methods, photoreceptors having a photoconductive layer formed on an electroconductive substrate and including a material such as selenium, selenium alloys or amorphous silicon; and photoreceptors including a photosensitive layer including an inorganic photoconductive material such as zinc oxide and cadmium sulfide which is dispersed in a binder resin, are well known. However, recently organic photoreceptors have been typically used because of having low costs, good designing flexibility and being non-polluting.
Specific examples of the organic photoreceptors include photoreceptors having the following photosensitive layers:    (1) photosensitive layers including a photoconductive resin typified by polyvinylcarbazole (PVK);    (2) photosensitive layers including a charge transfer complex typified by polyvinylcarbazole-2,4,7-trinitrofluorenon (PVK-TNF);    (3) photosensitive layers including a pigment dispersion typified by a phthalocyanine-binder system; and    (4) functionally-separated photosensitive layers using a combination of a charge generation material and a charge transport material.
Among these photoreceptors, the functionally-separated photoreceptors attract attention now.
The mechanism of forming an electrostatic latent image on a functionally-separated photoreceptor is as follows:    (1) when imagewise light irradiates a charged photoreceptor, the imagewise light is absorbed by a charge generation material in a charge generation layer after passing through a transparent charge transport layer located overlying the charge generation layer;    (2) the charge generation material absorbing light generates a charge carrier;    (3) the charge carrier is injected into the charge transport layer and transported through the charge transport layer (or the photosensitive layer) along an electric field generated by the charge formed on the surface of the photoreceptor; and    (4) the charge carrier neutralizes the charge on the surface of the photoreceptor, resulting in formation of an electrostatic latent image.
In the functionally-separated photoreceptors, a combination of a charge transport material having an absorption in an ultraviolet region and a charge generation material having an absorption in a visible region is known. However, the durability of such organic photoreceptors is not necessarily satisfactory.
Recently, copiers, facsimile machines and laser printers tend to be personalized. Such image forming apparatus need to have high durability and stability (i.e., being maintenance-free), to be small in size and to reuse toner collected in a cleaning section. Therefore, various developing devices having a new configuration have been used, namely, image forming processes have widely changed. Therefore, the requisite of durability increases more and more for the photoreceptors.
In order to impart good durability to a photoreceptor, it is essential to establish a technique by which image problems such as production of image defects and decrease of image density can be avoided even when a large number of copies are produced. It is well known that such image problems are caused by flaws and abrasion of the photoreceptor used. Therefore, in order to prevent such image problems, it is necessary to impart good durability to photoreceptors.
In attempting to improve the abrasion resistance of photoreceptors, the following proposals have been made.
(1) Abrasion Resistance Improving Methods by Increasing Mechanical Strength of Charge Transporting Layer
For example, in Japanese Laid-Open Patent Publications Nos. (hereinafter referred toas JOPs) 10-288846 and 10-239870, it is attempted to improve the abrasion resistance of a photoreceptor by using a polyacrylate resin as a binder resin.
In JOPs 9-160264 and 10-239871, it is attempted to improve abrasion resistance of a photoreceptor by using a polycarbonate resin as a binder resin.
In JOPs 10-186688, 10-186687, and 5-040358, it is attempted to improve abrasion resistance of a photoreceptor by using a polyester resin having a terphenyl skeleton, a polyester resin having a triphenyl methane skeleton, or a polyester resin having a fluorene skeleton as a binder resin, respectively.
In addition, JOPs 9-12637 and 9-235442 have disclosed photoreceptors having a charge transport layer including a binder resin including a polymer blend including a styrene elastomer.
However, in the methods mentioned above the photosensitive layers of the proposed photoreceptors have to include a large amount of a low molecular weight charge transport material in view of photosensitivity of the resultant photoreceptors. Low molecular weight charge transport materials typically make the resultant charge transport layers brittle, and thereby the durability of the photosensitive layer rapidly deteriorates as the content of the low molecular weight charge transport material included therein increases. Therefore, problems such that flaws are caused on the surface of a photoreceptor and the surface is seriously abraded occurs due to such low molecular weight charge transport materials present in the charge transport layer thereof. These problems cannot be avoided only by such methods using a specific binder resin in the photosensitive layers.
(2) Abrasion Resistance Improving Method Using Charge Transporting Polymer Material
JOP 7-325409 discloses a photoreceptor which uses a charge transporting polymer material instead of low molecular weight charge transport materials. It is supposed that the photoreceptor has relatively good abrasion resistance compared to the photoreceptor mentioned above in item (1) because the content of the resin component in the photoconductive layer can be relatively increased. However, good durability cannot be imparted to the resultant photoreceptor only by a method in which a charge transport polymer material is used instead of a low molecular weight charge transport material. The reason there for is considered to be that the abrasion of photoreceptors is not caused only by the mechanical stress applied thereto. In addition, it is generally hard to refine such charge transport polymer materials. Namely, if impurities cannot be removed from the photoreceptors, the residual potential of the photoreceptors gradually increases.
(3) Abrasion Resistance Improving Methods by Decreasing Friction Coefficient of Charge Transporting Layer
For example, JOPs 10-246978 and 10-20534 have disclosed photoreceptors which have a relatively low friction coefficient because of including a siloxane component in their charge transport layers. JOPs 5-265241 and 8-328286 have disclosed photoreceptors which have a relatively low friction coefficient because of including a particulate fluorine-containing resin in their charge transport layers.
In these proposals, it is attempted that the contact pressure applied to the surface of the photoreceptor can be decreased to improve the durability of the photoreceptor by decreasing the friction coefficient of the surface of the photoreceptor. However, lubricants used for decreasing the friction coefficient typically have a poor affinity for binder resins, and thereby almost all the lubricant included in a photosensitive layer tends to migrate from inside of the photosensitive layer to the surface of the photosensitive layer. Therefore, a problem in that the desired low friction coefficient cannot be maintained for a long period of time occurs.
In addition, when a binder resin having a lubricating property is used, the effect of improving abrasion resistance is not satisfactory, or rather such a binder resin deteriorates the abrasion resistance of the resultant photoreceptor because the resultant photoreceptor has a brittle photosensitive layer.
(4) Abrasion Resistance Improving Method by Providing Protective Layer
For example, In JOP 57-30846, 58-121044, 59-223443 and 59-223445, it is attempted to improve the mechanical strength of a photoreceptor is improved by providing a protective layer on the surface thereof, wherein the protective layer includes a metal or a metal oxide having a specific particle diameter and a specific particle diameter distribution.
The mechanical strength of the surface of a photoreceptor can be improved relatively easily by this technique, and therefore it is an effective method for improving the mechanical strength. However, when conventionally proposed protective layers are formed, other problems such as deterioration of resolution of the produced images and photosensitivity of the photoreceptor tend to occur. Therefore, it can be said that this technique is not a practical method of improving of the abrasion resistance.
It is well known that a heater is provided in an image forming apparatus to prevent dew condensation on the surface of a photoreceptor, resulting in prevention of deterioration of resolution of produced images. However, such a technique not only increases manufacturing costs of the image forming apparatus but also increase power consumption thereof. Therefore, it is not useful.
When a photoreceptor drum is used, a problem in that the photoreceptor vibrates when the photoreceptor is driven to be rotated. In addition, there is a case in which a photoreceptor produces a noise when the photoreceptor is charged. In order to prevent such problems, the photoreceptor drum is often padded with a rubber or a metal block. However, when such a heater is provided in the photoreceptor drum, such a pad cannot be included therein. Therefore it is preferable not to provide such a heater in photoreceptors.
(5) Abrasion Resistance Improving Methods by Reinforcing Charge Transporting Layer
For example, JOPs 46-782 and 52-2531 have disclosed photoreceptors, which includes a lubricating filler in the surface of the photoreceptors to attempt to improve the lubricating property of the surface, resulting in prolongation of the life of the photoreceptor.
In addition, JOPs 54-44526 and 60-57346 have disclosed photoreceptors, which includes a filler in an insulating layer or a photosensitive layer of the image bearing member (i.e., a photoreceptor) to attempt to improve the mechanical strength of the photoreceptor.
Further, JOPs 1-205171 and 7-261417 have disclosed layered photoreceptors which have an uppermost layer or a charge transport layer including a filler, to improve the hardness or lubricating property of the layer.
Furthermore, JOP 61-251860 discloses a proposal in which a hydrophobic titanium oxide powder is included in a photoreceptor in an amount of from 1 to 30 parts by weight per 100 parts by weight of the charge transport material used therein to improve the mechanical strength of the photoreceptor.
However, these photoreceptors have drawbacks in that the photosensitivity seriously deteriorates and/or residual potential seriously increases, and thereby the photoreceptors do not function as a photoreceptor. Therefore, these methods are not practical.
On the other hand, the following proposals have been made to improve the stability of the image qualities of images produced by photoreceptors:
(6) Image Stability Improving Methods Using Antioxidant
For example, JOPs 57-122444 and 61-156052 have disclosed photoreceptors which include an antioxidant in the photosensitive layers thereof.
(7) Image Stability Improving Methods Using Plasticizer
For example, JOPs 8-272126 and 8-95278 have disclosed photoreceptors which include a plasticizer in the photosensitive layer thereof.
The methods mentioned in (6) and (7) are effective at preventing deterioration of the charge properties of the photoconductive layers in repeated use. When these compounds are used for a charge transport layer which includes a binder resin and a charge transporting material having low molecular weight, the addition amount of these compounds is limited to a small amount because the charge transporting material is included therein at a high concentration. Therefore, these methods are not effective at improving the durability of the photoreceptor.
In addition, such charge transport layer typically has a relatively low glass transition temperature. When these compounds are added in the charge transport layer, the glass transition temperature of the layer further decreases to a temperature which is almost the same as the inside temperature of an image forming apparatus in which the photoreceptor is arranged. Therefore, other problems such as deformation of the photosensitive layer and toner adhesion to the photosensitive layer tend to occur. Therefore, these methods are also not effective at improving the durability of photoreceptors.
As mentioned in JOPs 8-272126 and 8-292585, the life of a photoreceptor depends on both the mechanical durability (i.e., whether the photoreceptor is hardly abraded or scratched) and electrostatic durability (i.e., whether the charge properties of the photoreceptor are hardly deteriorated in repeated use) thereof. Even when a photoreceptor has a good abrasion resistance, the life of the photoreceptor is deteriorated if the photoreceptor has a poor electrostatic durability (i.e., the electrostatic properties of the photoreceptor deteriorate when the photoreceptor is fatigued and/or the environment surrounding the photoreceptor changes), namely, the life of the photoreceptor depends on the electrostatic durability thereof.
Therefore, in order to prolong the life of a photoreceptor, both the mechanical durability and electrostatic durability of the photoreceptor have to be improved at the same time. In the above-mentioned conventional techniques, it is attempted to improve one of the mechanical durability and electrostatic durability. Namely, both the mechanical durability and electrostatic durability cannot be improved at the same time by these techniques.
In addition, there are many cases in which when it is tried to improve one of the mechanical durability and electrostatic durability of a photoreceptor, the other of the durabilities deteriorates (i.e., relationship between the mechanical durability and electrostatic durability is a trade-off relationship).
Thus, it can be said that the conventional techniques mentioned above are useful for improving a specific property, but are not a technique by which the life of the photoreceptor can be prolonged.
Thus, a photoreceptor having a good combination of mechanical durability and electrostatic durability has not yet developed.
Recently, electrophotographic image forming apparatus have good durability. In order to develop an image forming apparatus having a small size and low manufacturing costs, a photoreceptor having good durability is necessary. In addition, a heater which is used for preventing dew condensation on a photoreceptor and a controller using a potential meter should be omitted therefrom. Therefore, a photoreceptor which is hardly influenced by changes of environmental conditions such as temperature and humidity is needed.
Because of these reasons, a need exists for an electrophotographic photoreceptor which has good durability such that the photoreceptor is considered to be a durable part of an electrophotographic image forming apparatus and good resistance to temperature/humidity changes.